No abstract

Dc conductivity of the whole series of homoionic alkali exchanged montmorillonites is investigated by means of Complex Impedance Spectroscopy. Conductivity of the samples is measured at the dry state and at various water loadings between 0 and 6 absorbed water molecules per cation. Dc conductivity of all the dehydrated samples follows an Arrhenius behavior. In contrast, the hydrated samples exhibit a non-Arrhenius temperature dependence of dc conductivity that is fruitfully fitted by using the VTF's empirical law. It is then shown that the critical temperature, T VTF , increases with water loading until the later equals approximately 3 for the Li + and Na + samples and higher values for the K + , Rb + , and Cs + samples. So, it appears that the departure from the Arrhenius behavior is directly related to the number of water molecules in interaction with the alkali extra-framework cations. For water loadings higher than 3, activation energy for dc conductivity tends to values of about 0.2-0.3 eV for all samples independently on the alkali extra-framework cations. In contrast, activation energy appears to be very sensitive to the considered alkali sample for water loadings lower than 1 and, in that case, closely related to the dehydration enthalpy of the samples.

Dehydration enthalpy and dc conductivity obtained by means of thermogravimetric analysis and complex impedance spectroscopy respectively on three alkaline-earth, i.e. Mg 2+ , Ca 2+ , and Ba 2+ , exchanged montmorillonites with low water loading are reported. These data are compared with those previously reported on the same montmorillonite exchanged with the whole series of alkali cations, i.e. Li + , Na + , K + , Rb + , and Cs + . A comprehensive analysis of the interplay between the interlayer cations and the adsorbed water molecules is thus made possible regarding both the static, i.e. dehydration enthalpy, and dynamics, i.e. dc conductivity, points of view. It is pointed out that dehydration thermodynamics strongly depends on the cation electrostatic potential energy, whereas the dc conductivity temperature behavior, which turns out to be non-Arrhenian for the most hydrated samples can be remarkably well classified using the cation Jones-Dole viscosity B coefficient.